As I read some of the comments regarding the open vias
          problem, I think there is some confusion which I'd like to
          clear up.  Regarding tolerancing microvias with no bottom
          tolerance (i.e., they may plug shut), the plugging generally
          occurs during the solder levelling operation, not plating.
          Board fabricators must walk the tightrope between keeping
          holes open and not blowing too much solder off of surface
          mount pads when adjusting air knife placement and pressure.
          Another issue is using a small enogh drill diameter so that
          hole breakout is minimized.  Board fabricators generally
          like some slack with vias in using a smaller drill size than
          the nominal callout of the via hole diameter in order to
          keep the hole in the pad.  Aspect ratios (bd. thickness to
          dill size) of 7:1 or greater are not uncommon in volume
          environments these days.  My company has just installed new
          PAL plating lines in our volume shops to meet such
          challenges.  The PAL folks tell us that we should be able to
          plate boards with aspect ratios of 10:1!
               The fallout rate experienced by the original author
          strikes me as unusually high.  One possible way that they
          might have escaped the E.T. net is if they were intermittent
          opens which may show up only when the board is flexed or
          stressed.  Gary Ferrari's comments about supplying and
          requesting IPC-D-356 net lists should be well taken.  This
          is an extra measure which should guarantee 100% electrical
          test on the part of the board fabricator.  At the very
          least, it assures that the design intent and the Gerber
          supplied match.
               The bottom line is that I'd like to dispel any notion
          that allowing vias to plug shut causes a less reliable
          connect.  Hole sections should be done by the board
          fabricator to get to the root of the real problem.
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation
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